This invention relates to transmissions, and more particularly to an improved gear change mechanism for a transmission that produces a precision drive that is especially adapted for high speed machine tool applications.
Typical prior art gear change mechanisms consist generally of two types. In one type, a shifting fork and sliding gear arrangement is utilized and in another, constant mesh gearing using clutch and/or brake shifting is employed. As rotational speeds increase in the gear drive train these mechanisms cause one or more of the following undesirable traits. When using shifting forks, there is introduced a moment loading to the shifting gear, so that the shaft to gear fit must be comparatively loose, causing an out of balance condition as the gear is rotated. Further, shifting forks contact the shifting gear off the center of gear rotation, causing a sliding action between the gear and the shifter. Even with the addition of rollers to the shifting fork, the possibility of sliding, (power loss) occurring at high speed increases. Constant mesh gearing using clutchbrake shifting results in all gears always remaining in mesh. Such arrangement consumes additional power and adds inertia to the gear system. Also, clutch-brake units rated for high rotational speed are not readily available and may require additional control circuitry.
In U.S. Pat. No. 4,449,866 assigned to the assignee of this invention, there is described a servo-controlled spindle drive system utilizing a piston and cylinder mechanism for shifting a gear shaft 26 and its associated cluster gear 24. However, even though this particular gear change servo system does not utilize the previously referred to shifting fork and sliding gear arrangement or the constant mesh gearing utilizing clutch and/or brake shifting, it does have disadvantages in that it does not provide for precision fits and precise gear movements that especially adapt the transmission for high speed operation under relatively heavy loads while maintaining its accuracy for automatic operation under the control of a numerical control circuit.
It is an advantage of the present invention to provide an improved gear change mechanism which shifts the gears of a transmission with a force applied uniformly about the axis of the gear to prevent any misalignment during shifting so that the gear can be fitted onto its supporting shaft with extremely close tolerance and yet enable the gear to shift easily along the shaft without binding.
It is another advantage of the present invention to provide a shifting mechanism which reduces wasted power in the mechanism to an absolute minimum.
It is yet another advantage of the invention to provide an improved gear change mechanism that is especially adapted for high speed operation under relatively high load while transmitting the power with extreme accuracy for automatic operation under the control of a numerical control circuit.
It is still another advantage to provide a gear change mechanism which minimizes heat or other disturbances in a gear change system which includes a transmission and is operated at high speed.